The study of the composition and properties of enamel fluid as a function of external conditions will be continued. By using some of our new techniques, we hope to obtain not only equilibrium values of the composition but also an accurate measurement of the rate change of the relevant compositions and therefore determine the diffusion fluxes within enamel. We may thus find which ions are rate controlling in the decay process. Three new projects will also begin: 1. We will modify the permselective properties of enamel and perform some of our previous experiments in order to deduce the effect of plaque and pellicle on enamel fluid composition. 2. The plaque project performed in conjunction with the Forsyth Dental Center will continue in order to obtain the calcium, phosphate, fluoride and pH levels directly in contact with carious and sound enamel. 3. The diffusion of chloride and fluoride in enamel will be extensively studied in order to obtain information on the transport rate and reaction of fluoride inside carious and sound enamel.